Extruded ceramic tubes or monoliths are used for the filtration and separation of gasses and liquids. Such uses include the filtering of beer to remove active yeast and the filtering of water to remove particulate matter. Another use for ceramic bodies is in the separation of gas mixtures, such as the separation of hydrogen from methanol. An example of a body useful for this application is an extruded and fired alumina or mullite tube containing a zeolite membrane. A further improvement of this is to use an extrudate containing a multiplicity of cylindrical channels oriented parallel to the direction of the extrusion (typically the long axis). Typically seven, eleven, nineteen, twenty-eight, thirty-one, or thirty-seven parallel cylindrical channels are arranged in a hexagonal manner or in increasing concentric rings as shown in FIGS. 1 and 1a as a and c.
The liquid or gas which has passed through the filter is commonly referred to as the filtrate or permeate. One difficulty with this geometry is that the filtrate must travel a relatively long and tortuous path from any interior channel (e.g. channel a in FIG. 1a) to the exterior of the extrudate (e.g. b in FIG. 1a). Channels near the exterior (e.g. channel c in FIG. 1a) also suffer interference from the permeate from the interior channels. As a result, when more than one cylindrical channel, or more than one ring of channels is present, there can be a high back pressure and a reduced flux of filtrate per unit area of filter. The present invention alleviates this difficulty.
Often the extruded body is coated in order to form a membrane which differs from the bulk of the extruded body. This membrane may be formed from polymers, ceramics, glasses, and metals with special porosity or chemical activity. Often these membranes are formed from particulate containing slips. An additional difficulty with the standard multi-channel geometries is that they are difficult to coat with particulate containing slips since coating involves the flow of liquid through the ceramic body. Resistance to flow of the liquid from the slip can be detrimental to coating uniformity and adhesion.
Various filtration, and separation devices are described in U.S. Pat. Nos. 5,108,601, 4781,831, 5,009,781, and 4,222,874. Generally these devices suffer the disadvantages of either inefficient flow through and removal of the filtrate, or relatively complex fabrication.
For example, U.S. Pat. Nos. 5,108,601, 4,781,831, and 5,009,781 to Goldsmith relate to cross-flow filtration devices formed of porous material, for separating a feed stock into filtrate and retentate. These devices define a number of passageways extending longitudinally from the feed end face to a retentate end face of the structure, and which can serve as membrane supports. In general, these devices have a number of filtrate chambers distributed among the passageways for removal of the filtrate to ensure a favorable pressure drop from any passageway wall to a nearby chamber. The design and fabrication of these devices have some limitations. For example, Goldsmith shows several rows of passageways to one filtrate chamber. This arrangement results in significant back pressure during a filtration operation, the flow decreasing as the material passes through the device. This variation in flow is not acceptable. To compensate for this back pressure, the porosity and pore size of these devices is relatively large. However, larger pores are not desirable when membranes are applied or when the separation properties of the device are dependent on pore size. It would be advantageous to vary the pore size and porosity of the device without sacrificing filtration or separation efficiency (i.e. the ratio of the amount of filtered material to the amount of material passing through the device without being filtered or separated). Goldsmith does not make use of all of the surface area because of the flow limitations of his devices. Another difficulty with the Goldsmith devices is in the complexity of fabrication.
Connelly, in U.S. Pat. No. 4,222,874, discloses a relatively small tube for internal passage of the permeates. Connelly discloses multiple permeate ducts (i.e. conduits) but these only communicate through the end faces of the device. Connelly provides only a few, widely spaced conduits.